According to one book on MJER I've read there was a historical preference for only one. This is really a question you should ask your instructor. If the tsuka is made properly, one should be all you need.

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The amount of material lost is minor and supported on both sides, so unless the blade is trash to begin with, don't worry about losing strength to the holes. Your tip is still the weakest link, not the tang.

Also, if the blade is well made, the mekugi are mostly decorative, the sword should hold together without them (like a good, new sheath should hold the knife when it's completely upside down). As it wears, a single mekugi will be able to hold it.

Having two or three might reduce rattling when the grip starts to wear on the inside, but if gets to that point (it shouldn't without years and years of heavy use or termites), then you should have new furnishings made anyway. Of course, there is the comfort of having backup systems, but I haven't heard of mekugi breaking in the past.

Quote:Also, if the blade is well made, the mekugi are mostly decorative, the sword should hold together without them (like a good, new sheath should hold the knife when it's completely upside down). As it wears, a single mekugi will be able to hold it.

These statements are both true and not true. A properly made handle will hold the tang in it tightly. The Japanese sword is designed to pressure fit together. However, the mekugi are most definitely NOT decorative. Their job is not to keep the sword from coming apart, but to pressure the tang into the handle. This is why mekugi are tapered instead of straight. In a well made custom tsuka (handle), a single mekugi will hold things in correct alignment just fine. However, in production type swords, which aren't made by highly experienced craftsmen intent on taking their time and doing their best, it is a good idea to have a back-up mekugi because it just may end up the only thing keeping the blade from flying out.

There are problems with extra mekugi though. To picture it, you have to imagine what the mekugi is doing. The hole in the handle and the hole in the tang should line up in such a way that fully seating the mekugi will push the tang toward the back of the handle. When you have two mekugi, they both should do this. Invariably though, only one will press the tang backwards, and the other will just sit, or will press the tang forwards instead! The reason being that it's twice as hard to perfectly align four holes (the holes through the handle and the holes through the tang) instead of two. Now, add a third mekugi, and your craftsman is expected to perfectly align all six holes. It isn't likely to happen.

One mekugi is not adequate for most people. There was an incident in Japan where an iaidoka had forgotten to replace the mekugi and the blade flew out and killed a child.

I'm sure similar things happened in the past but went unrecorded for the history books.

2 mekugi is what most people ought to have in their blades. 1 mekugi is for genuine swords made in the traditional way and used for traditional purposes only, with the utmost care being taken at every step.

It sounds strange, but the bamboo mekugi are far more reliable than the people who wield the blades.